Dielectric films for narrow gap-fill applications

ABSTRACT

A colloidal suspension of nanoparticles composed of a dense material dispersed in a solvent is used in forming a gap-filling dielectric material with low thermal shrinkage. The dielectric material is particularly useful for pre-metal dielectric and shallow trench isolation applications. According to the methods of forming a dielectric material, the colloidal suspension is deposited on a substrate and dried to form a porous intermediate layer. The intermediate layer is modified by infiltration with a liquid phase matrix material, such as a spin-on polymer, followed by curing, by infiltration with a gas phase matrix material, followed by curing, or by curing alone, to provide a gap-filling, thermally stable, etch resistant dielectric material.

FIELD OF THE INVENTION

[0001] The present invention relates generally to dielectric materialsfor use in semiconductor devices, and, more specifically to dielectricmaterials, prepared from colloidal dispersions, that have high thermalstability and etch resistance and completely fill narrow gaps.

BACKGROUND

[0002] In order to provide integrated circuits (ICs) with increasedperformance, the characteristic dimensions of devices and spacings onthe ICs continue to be decreased. Fabrication of such devices oftenrequires the deposition of dielectric materials into features patternedinto layers of material on silicon substrates. In most cases it isimportant that the dielectric material completely fill such features,which may be as small as 0.01 to 0.05 μm or even smaller in nextgeneration devices. Filling such narrow features, so-called gap filling,places stringent requirements on materials used, for example, forpre-metal dielectric (PMD) or shallow trench isolation (STI)applications. The pre-metal dielectric layer on an integrated circuitisolates structures electrically from metal interconnect layers andisolates them electrically from contaminant mobile ions that degradeelectrical performance. PMD layers may require filling narrow gapshaving aspect ratios, that is the ratio of depth to width, of five orgreater. After deposition, the dielectric materials need to be able towithstand processing steps, such as high temperature anneal, etch, andcleaning steps.

[0003] Dielectric materials are commonly deposited by chemical vapordeposition (CVD) or by spin-on processes. Each of these approaches hassome limitations for filling very narrow gaps. Plasma enhanced chemicalvapor deposition (PECVD) processes provide high deposition rates atcomparatively low temperatures (about 400° C.). The main drawback isthat PECVD processes have a lower deposition rate inside a gap than atother locations on a surface. The differential deposition rates cancreate structures overhanging a gap opening, leading to voids within thegap. Typically, for spacings less than 0.25 μm, depending on the aspectratio, it is difficult to achieve void-free gap fill using standardPECVD approaches.

[0004] Phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG)are commonly used for premetal dielectric applications. The films areusually deposited using atmospheric pressure CVD (APCVD),sub-atmospheric pressure CVD (SACVD) or low pressure CVD (LPCVD).Depending on the process conditions and precursors used these methodscan achieve an almost conformal coating. Gap-fill is achieved by apost-deposition reflow process in which the material is treated at hightemperatures, typically 800-1200° C. The inclusion of phosphorous andboron, in particular, the boron, in the glass lower the glass transitionand flow temperatures. However, the use of CVD followed by reflow infuture advanced devices will be limited by the high thermal budgetrequired for the reflow process, which is not compatible with certainmaterials and processes, such as cobalt silicide used at the contactlevel. For very narrow gaps, less than 0.2 μm, there is an increasingrisk that voids may remain, even after high temperature processing.

[0005] Some workers have used high density plasma chemical vapordeposition (HDP CVD) to improve gap-fill of PSG and BPSG. In the highdensity plasma process, deposition and etching occur simultaneously.Etching is most efficient at the top comers of narrow openings, therebycompensating for a lower deposition rate inside the gap. HDP CVDdeposition does not require high temperature processing, although ananneal step can be used if a denser film is desired. The HDP CVD processhas the drawback that for narrower structures, lower deposition to etchratios have to be used resulting in a relatively slow overall fillingrate. Improved gap-fill may also require modifications in the design ofdevice features, such as rounded comers and sloped sidewalls. Finally,there is also a concern about plasma damage to the device during HDP CVDprocessing.

[0006] Spin-on glasses and spin-on polymers such as silicates,siloxanes, silazanes or silisequioxanes generally have good gap-fillproperties. The films of these materials are typically formed byapplying a coating solution containing the polymer followed by a bakeand thermal cure process. The utility of these spin-on materials may belimited, however, by material shrinkage during thermal processing.Thermal shrinkage is a key consideration for materials which have towithstand high process temperatures, such as materials used forpre-metal dielectric and/or shallow trench isolation applications, whichmay involve process temperatures exceeding 800° C. High shrinkage canlead to unacceptable film cracking and/or formation of a porousmaterial, particularly inside narrow gaps. Cracked or porous materialmay have an undesirably high wet etch rate in subsequent process steps.

[0007] Thus there remains a need for a dielectric material that providesvoid-free gap-fill of narrow features at processing temperatures lessthan the reflow temperatures used currently. The gap-filling materialsneed to have high thermal stability and reasonable resistance to etchingsolutions to survive subsequent processing steps.

SUMMARY

[0008] A colloidal dispersion of particles composed of a dense materialdispersed in a solvent is used in forming a gap-filling dielectricmaterial with low thermal shrinkage. The particles are preferably ofnanometer-scale dimensions and are termed nanoparticles. The densematerial is either a dielectric material or a material convertible to adielectric material by oxidation or nitridation. The dielectric materialis particularly useful for pre-metal dielectric and shallow trenchisolation applications. Oxides and nitrides of silicon, oxides andnitrides of aluminum, and oxides and nitrides of boron are useful asnanoparticle materials. Colloidal silica is particularly useful as thecolloidal dispersion. The dielectric material optionally includes dopantspecies such as arsenic, antimony, phosphorous, or boron.

[0009] According to the methods of forming a dielectric material, thecolloidal dispersion is deposited on a substrate and the deposited filmis dried forming a porous intermediate layer. The intermediate layer ismodified by infiltration with a liquid phase matrix material, followedby curing, where, in all cases, curing includes optionally annealing, byinfiltration with a gas phase matrix material, followed by curing, or bycuring alone, to provide a gap-filling, thermally stable, etch resistantdielectric material.

[0010] Infiltrating matrix materials applied in the liquid phase arespin-on polymers, including oligomers and monomers, that can beconverted to silica or similar ceramic materials on high temperaturecure, optionally in the presence of oxygen or steam. The matrixmaterials include, but are not limited to, silicates, hydrogensilsesquioxanes, organosilsesquioxanes, organosiloxanes,silsesquioxane-silicate copolymers, silazane-based materials,polycarbosilanes, and acetoxysilanes. The liquid matrix materialsoptionally include dopant species such as arsenic, antimony,phosphorous, or boron. In liquid phase infiltration, a coating solutionof the matrix material is applied on the colloidal film.

[0011] Gas phase infiltration uses chemical vapor deposition (CVD)methods under conditions in which impinging molecules have a lowsticking coefficient and/or high surface diffusion to avoid sealing atop surface of a narrow gap before bulk porosity of the intermediatelayer is reduced. The CVD deposited materials optionally include dopantspecies. Other gas phase deposition processes, such as atomic layerdeposition, may also be used for gas phase infiltration.

[0012] The dried intermediate layer or the infiltrated intermediatelayer is cured, for example in a furnace at temperatures of betweenabout 600 and 800° C. or by rapid thermal processing, for example, attemperatures of between about 700 and 900° C. Optionally, one or morebake steps at temperatures, for example, between about 75 and 300° C.precede the curing process. In addition, the cure may be followed by ahigher temperature annealing step. Curing, and optionally annealing,induces sintering of the nanoparticles and reflow of the infiltratedmatrix material. Inclusion of dopant species in the nanoparticles or inthe matrix materials lowers the reflow temperature.

[0013] Films prepapred according to the present invention fill narrowgaps, less than 100 nm and as small as 50-60 nm in width, without voidsor cracks. They do not show delamination of cracking even after heattreatment at 900° C. The cured films have minimized open porosity asevidenced by their resistance to etchant solutions in filled gaps.Further, the average etch rate of cured films on blanket wafers is onthe same order as the average etch rate of silicon dioxide filmsproduced by chemical vapor deposition. Thus, films prepared according tothe present processes are advantageously used as pre-metal dielectricand shallow trench isolation materials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIGS. 1-3 are flow diagrams of processes of forming a dielectricmaterial according to embodiments of the present invention.

[0015]FIG. 4 is a pre-metal layer filled with a dielectric materialformed according to an embodiment of the present invention.

[0016]FIG. 5 is a shallow trench isolation structure filled with adielectric material formed according to embodiments of the presentinvention.

DETAILED DESCRIPTION

[0017] Methods of forming a gap-filling dielectric material with lowthermal shrinkage make use of a colloidal dispersion of dense particles.A coating solution of the colloidal dispersion is deposited on asubstrate to form a film, and the film is intentionally modified by oneor more approaches. The dielectric material is beneficially used forpre-metal dielectric and shallow trench isolation applications.

[0018] Key to the present invention is the nature of the colloidaldispersion. Colloids are generally defined as systems in suspension inwhich there are two or more phases. In the colloidal suspension, one ofthe phases, termed the dispersed phase, is distributed in the otherphase, termed the continuous phase. A familiar type of colloid consistsof dispersions of small particles in a liquid.

[0019] According to one aspect of the present invention, a colloidaldispersion of nanometer scale particles, termed nanoparticles, composedof a dense material dispersed in a solvent is used. The dense materialis either a dielectric material or a material convertible to adielectric material by reaction with oxygen or nitrogen. In addition,the physical size of the nanoparticles needs to be substantiallyunchanged by thermal processing. The size of the nanoparticles shouldnot be reduced by more than 10% when exposed to temperatures of about700° C. In the event the nanoparticles are composed of a material suchas silicon or aluminum, convertible to a dielectric material byoxidation or nitridation, the particle size may increase somewhat duringcuring in the presence of an oxygen- or nitrogen-bearing species.

[0020] Silicon and aluminum and refractory oxides and nitrides ofsilicon and aluminum are useful nanoparticle materials. Additionaluseful materials include nitrides, such as boron nitride and galliumnitride, and also boron oxide and boron carbide. Suitable dense,silicon-containing materials for use as nanoparticles include silica,silicon, silicon nitride, silicon oxynitride, and combinations andmixtures thereof. For example, colloidal silica is advantageously usedas the colloidal dispersion. Methods for forming colloidal silica areknown in the art as described, for example, in U.S. Pat. No. 3,634,558and in Van Helden et al., (J. Colloid Interface Sci. 81, 354 (1981)),both of which are incorporated herein by reference. In addition,colloidal silica is available commercially. A general criterion is thatthe nanoparticles experience little or no chemical change during a hightemperature cure process with the exception of oxidation or nitridation,as discussed above. However, as long as the low shrinkage criterion ismet, nanoparticles may additionally contain small quantities of siliconpolymers such as hydridosilsesquioxanes, organosiloxanes,organosilsesquioxanes, and perhydrosilazanes.

[0021] The nanoparticles have a characteristic dimension between about 2nm and about 50 nm. The colloidal dispersion has excellent gap-fillingcapability, limited only by the particle size. For any specificapplication, the particle size is chosen to be smaller than the width ofthe opening to be filled. The size distribution of the nanoparticles maybe monodisperse, bimodal, or polydisperse. Bimodal distributions may betailored to provide a higher packing density of nanoparticles, in whichsmaller particles fit into voids generated by packing of largerparticles. The nanoparticles are dispersed in an organic solvent orinorganic solvent, such as an aqueous solvent or solvent mixture, or ina supercritical fluid. Suitable organic solvents include solventscommonly used in coating solutions of spin-on polymers, such asmethanol, ethanol, isopropyl alcohol, methylisobutylketone,cyclohexanone, acetone, and anisole, to name only a very few. The solidcontent of nanoparticles in the colloidal dispersion typically rangesfrom as little as 0.5 weight % to as much as 20%. Higher or lowerconcentrations may be used to adjust the coating thickness. Additionaladditives such as surfactants or binders may also be present in thedispersion. Infiltration matrix materials described below are useful asbinders when added to the dispersion in small quantities, for example ina ratio of nanoparticle to binder greater than about 10:1.

[0022] According to another aspect of the present invention, thenanoparticles include dopant species such as arsenic, antimony,phosphorous, or boron. Including such dopants may enhance materialproperties of the film formed from the colloidal dispersion. Forexample, dopants are introduced to increase mobile ion gettering and tolower the glass transition temperature. Boron is used, among otherreasons, to produce a dielectric material with increased etchresistance.

[0023] A film of the colloidal dispersion is typically formed on asubstrate by spin coating. Other methods known in the art for applyingcoating solutions such as dip coating or spray coating may alternativelybe used. The coated film is dried evaporating the solvent in thedispersion. The coated film may be dried during the spin-coatingprocess, for example by a fast spin, or by a rest period following thedispense step. Alternatively, the coated film may be heat treated bymethods such as lamp heating, baking on a hot plate, at one or moretemperatures between about 75 and 300° C., or by other methods known inthe art. In addition to evaporating the solvent, the heat treatment mayserve to keep the particles attached to the substrate, so that they arenot removed/rinsed away during the subsequent infiltration. The heattreatment may also have the effect of reacting the optional bindermaterial. The film thus formed on the substrate is typically porous innature with an open pore structure. For applications such as PMD or STIlayers, it is desirable to minimize porosity of the film in order tominimize thermal shrinkage and moisture sensitivity and to maximizethermal stability and etch resistance. It is also desirable to changethe open pore structure to a closed pore structure. Eliminating openporosity improves the chemical resistance such as etch resistance of thematerial during subsequent processing steps. To minimize open porosity,a film formed from the colloidal suspension is modified by one or moreapproaches. The processes include infiltration with a matrix materialapplied in the liquid phase, infiltration with a matrix material appliedin the gas phase, and cure/anneal processes.

[0024] Infiltrating matrix materials applied in the liquid phase arespin-on polymers that can be converted to silica or similar ceramicmaterials during a cure process. An example is a high temperature cure,optionally in the presence of oxygen or steam. The term spin-on polymer,as used here, includes oligomers and monomers, as well as polymers. Thematrix materials include, but are not limited to, silicates, hydrogensilsesquioxanes, organosilsesquioxanes, organosiloxanes,organhydridosiloxanes, silsesquioxane-silicate copolymers,silazane-based materials, polycarbosilanes, and acetoxysilanes. Suitablecommercial matrix materials include silicates of the T11 and T14 series,the spin-on-glass Accuspin™, and the organohydridosiloxane HOSP™, allprovided by Honeywell International, Inc. (Morristown, N.J.).

[0025] In liquid phase infiltration, a coating solution of the matrixmaterial is applied on the colloidal film, typically by spin coating,although alternative application methods may be used. Low molecularweight matrix materials have the benefit of more easily penetratingnarrow spaces between the nanoparticles. Matrix materials with molecularweights ranging from several hundreds to multiple thousands of atomicmass units (amu) may be desirable. Infiltrating molecules withhydrodynamic diameters less than 1-2 nm are advantageous to infiltratesmall pores. Solid content of the infiltrant solution may range fromnearly 100% solid to as low as 2% solid. The infiltrating matrixmaterial occupies space between the nanoparticles, minimizing openporosity. In addition, depending on the molecular weight of the matrixpolymer, the matrix material may also form an overlayer on top of theinfiltrated colloidal layer, as described, for example, in Example 5below.

[0026] In some embodiments, dopant species, such as arsenic, antimony,phosphorous, or boron, as described above, are included in the matrixmaterials. Dopants are introduced, for example, to increase mobile iongettering and to lower the glass transition temperature of thegap-filling dielectric material. The introduction of phosphorous hasbeen observed to provide better gap-filling properties and boron isused, among other reasons, to produce a dielectric material withincreased etch resistance and to provide coating solutions for applyingthe dielectric material with longer shelf life. Dopants may be providedfor other reasons, as well. In particular, boron and phosphorous dopedsilicates and silsesquioxanes are useful doped matrix materials.Suitable phosphorous doped silicates include the phosphosilicateproducts P062A, P082A, P112A, P064A, P084A, and P114A, all provided byHoneywell International, Inc. (Morristown, N.J.). A boron dopedsilsesquioxane is prepared, for example, by adding a solution of boronoxide in isopropyl alcohol to a silsesquioxane solution as describedbelow in Example 3. A boron and/or phosphorous doped silicate isprepared by including a boron and/or phosphorous precursor in thesynthesis of the silicate polymer. Suitable phosphorous precursorsinclude but are not limited to P₂O₅, H₃PO₄, and trialkylphosphates, suchas trimethylphosphate, and triethylphosphate. Suitable boron precursorsinclude but are not limited to B₂O₃, H₃BO₃, and trialkylborates, such astrimethylborate, and triethylborate. The synthesis of boron andphosphorous doped silicate using P₂O₅ and B₂O₃ as reactants is describedbelow in Example 7. Alternatively, the matrix material may comprise amixture of the undoped spin-on polymer and the boron and/or phosphorousprecursors described above. Additional dopant-containing moleculesuseful in dopant/matrix material mixtures include phosphazenes,borazenes, and borophosphates.

[0027] In an alternative embodiment, the matrix material is included asa component of the colloidal dispersion. Pre-mixing the colloidaldispersion and the matrix material has the advantage of eliminating theprocess step of applying a coating solution of the matrix material. Inthe case of pre-mixing, the fraction of added matrix material is suchthat the majority of the volume in the gap is occupied by thenanoparticles in order to minimize thermal shrinkage of the dielectricmaterial. For example, the ratio of nanoparticles to matrix material isgreater than or about 1:1. Note that when the matrix material is used asa binder in the colloidal dispersion, the ratio of nanoparticles tomatrix material is much greater than when the matrix material ispremixed in the colloidal suspension.

[0028] Gas phase infiltration with matrix materials provides a secondapproach to densification. Gas phase infiltration processes includechemical vapor deposition (CVD) and atomic layer deposition. For use asan infiltration process, CVD conditions are adjusted to avoid sealing atop surface before bulk porosity is reduced. The CVD process isperformed, therefore, in a reaction rate limited process regime,achieved, for example, by conditions in which impinging molecules have alow sticking coefficient and/or high surface diffusion. The stickingcoefficient is the probability of an arriving molecule to react on thesurface. A sticking coefficient of 1 means that every molecule willremain on the surface. For successful infiltration using CVD, thesticking coefficient has to be smaller than 0.01. For a thermal CVDprocess this can usually be achieved by choosing a depositiontemperature lower than the standard process temperature. For typical CVDdeposition of material layers on flat surfaces, such conditions wouldresult in unacceptably low deposition rates. However, in the presentapplication to deposition on a highly porous film, the surface area canbe many times larger than the flat surface substrate area, enhancing theeffective deposition rate to practical levels. An exemplary gas phasedensification process is CVD deposition of tetraethoxysilane (TEOS) andoxygen at a temperature between about 400 and 600° C. Infiltrationdeposition temperatures are significantly lower than the about 600 to700° C. process conventionally used for CVD of the TEOS/oxygen system.Dopants, as listed above, can be included in the CVD infiltrationprocess by including dopant precursor gases along with the CVD reactantgases. Examples of dopant precursors may include boron oxides,triethylborate, alkylboranes, and diborane for boron doping, andtriethylphosphate, trimethylphosphate, and phosphine for phosphorousdoping.

[0029] Atomic layer deposition (ALD) provides an alternative gas phaseinfiltration process. In ALD, each atomic layer is deposited byalternatively supplying a reaction gas and a purging gas. Thus, ALD isan excellent approach to forming a conformal, uniform coating of theentire pore surface structure of the colloidal dispersion layer. Anexample for an ALCVD process for a depositing Al₂O₃ uses Al(CH₃)₃ andwater vapor as precursors. An ALCVD coating consisting of less than20-30 atomic layers is beneficially used in the present invention.Larger numbers of atomic layers may also be used.

[0030] Curing processes, which may include thermal processing orannealing methods, such as electron beam annealing or ion beamannealing, or combinations thereof, provide a third approach tomodification of the open pore structure. The curing process may beapplied to the as-deposited colloidal dispersion, or to the film after aliquid phase or gas phase infiltration process has been performed. Allmethods of forming a gap-filling dielectric material according to thepresent invention include a curing step. Optionally, one or more bakesteps at temperatures between about 75 and 300° C. may precede thecuring process. The film is cured in vacuum or in an atmosphere ofcommonly used gases such as nitrogen, oxygen, ozone, steam, ammonia,argon, carbon monoxide, carbon dioxide, nitrous oxide, nitric oxide,helium, hydrogen, forming gas, or mixtures thereof. For nanoparticles ofa material, such as silicon, convertible to a dielectric by oxidation ornitridation, a curing atmosphere containing an oxygen or nitrogenbearing species is used.

[0031] An exemplary curing process is curing in a furnace attemperatures between about 600 and 800° C. in a nitrogen/oxygenatmosphere for a time period less than about 2 hours. Alternatively, thefilm is cured by rapid thermal processing (RTP) at a temperature betweenabout 700 and 900° C. for a period of about 10 seconds to 5 minutes.Optionally, particularly for processes in which neither thenanoparticles nor the matrix material are doped materials, curing may befollowed by a higher temperature thermal annealing step. An exemplaryannealing process uses temperatures between about 800 and 1000° C. in anitrogen atmosphere. In the present processes, curing modifies thedielectric film everywhere on a substrate including inside narrow gapsand in open regions.

[0032] The present processes are used to deposit dielectric materials onsubstrates in the fabrication of integrated circuit devices. Integratedcircuit devices include but are not limited to silicon based devices,gallium arsenide based devices, opto-electronic devices, focal planearrays, photovoltaic cells, and optical devices. As is well known,integrated circuit devices generally include a substrate, conductivecircuit lines, and dielectric material. In addition, barrier layers,etchstop layers, and conducting gates, such as polysilicon gates, areincluded in the devices. The interconnected circuit lines function todistribute electrical signals in the device and to provide power inputto and signal output from the device. Integrated circuit devices willgenerally include multiple layers of circuit lines which areinterconnected by vertical metallic studs, that is metal-filled vias.Suitable substrates include, but are not limited to, silicon, silicondioxide, glass, silicon nitride, ceramics, and gallium arsenide. As usedherein, substrates refers to any of the layers, planarized or havingtopography, including, semiconducting wafers, dielectric layers, gates,barrier layers, etchstop layers, and metal lines found in integratedcircuit devices.

[0033] The principal methods of forming a gap-filling dielectricmaterial according to the present invention are summarized in theprocess flow diagrams of FIGS. 1-3. FIG. 1 describes a process 10 offorming a film without infiltration with a matrix material. A colloidaldispersion of nanoparticles is deposited on a substrate, at step 12,followed by an optional bake step 14. The nanoparticles may be doped orundoped. The final step of process 10 is a cure process 16 as describedabove. Process step 16 in FIGS. 1-3 implicitly includes an optionalhigher temperature anneal. FIG. 1 also describes the alternative processof forming a dielectric material in which matrix material is premixedinto the colloidal dispersion at step 12. When depositing a premixeddispersion, bake step 14 is included in the process.

[0034]FIGS. 2 and 3 describe processes 20 and 30 including infiltrationwith a liquid matrix material and by a gas phase process, respectively.In processes 20 and 30, deposition of the colloidal dispersion, step 12,is followed by an optional bake process, 14. For liquid phaseinfiltration, process 20, at step 26, a coating solution of the matrixmaterial is applied over the film formed from the colloidal dispersion.The coating solution at step 26 may contain dopants including but notlimited to boron and/or phosphorous. Step 26 may optionally includemultiple applications of a coating solution of matrix material. Coatingthe matrix material is followed by a bake step, 28, and cure step 16. Inthe gas phase infiltration process, 30, after depositing the colloidaldispersion at step 12 and an optional bake process at step 14, a matrixmaterial is deposited on the nanoparticles by chemical vapor deposition,at step 36. Materials containing dopants such as boron and phosphorousare beneficially used at step 36. A cure step 16 completes process 30.Processes 20 and 30 provide a composite gap-filling dielectric materialconsisting of the matrix material surrounding nanoparticle fillermaterial.

[0035] The present material is particularly useful for filling narrowgaps with a dielectric material in a pre-metal layer and for fillingtrenches in shallow trench isolation structures, as illustratedschematically in FIGS. 4 and 5, respectively. Pre-metal layer 40 in FIG.4 includes poly-silicon gates 43, and a barrier layer 44, on a substrate45. Gap 42 can be even narrower than the “minimum feature size,” ascommonly defined by the limitations of photolithographic methods. Theparticular embodiment illustrated in FIG. 4 shows dielectric layer 46,including nanoparticles 47, topped by an overlayer 48 of curedinfiltrating matrix material. The matrix material also occupies spacebetween the nanoparticles (not depicted). Using a colloidal silica toprovide the nanoparticles and boron-doped silsesquioxane as theinfiltrating matrix material, gaps smaller than 100 nm, as small as50-60 nm in width (0.05-0.06 μm), have been completely filled withoutdelamination, as described in Example 5 below.

[0036] An exemplary shallow trench isolation structure 50 depicted inFIG. 5 includes substrate 52, pad oxide layer 53, hard mask 54, lineroxide 55, and trench 56 filled with the present dielectric material. Acharacteristic dimension 57 of the shallow trench is typically 1-2 timesthe “minimum feature size.” Thus, STI applications also require amaterial that fills narrow openings. For STI use, nanoparticles aretypically composed of silica, silicon, or mixtures thereof. Infiltrationis performed in the liquid phase or in the gas phase, preferably with anoxide or with matrix material that can form an oxide, such as SiO₂, orAl₂O₃. For gas phase infiltration, silicon oxide may be deposited usingstandard CVD precursors, which include but are not limited totetraethoxysilane (TEOS) and oxygen or ozone, or silicon may bedeposited using precursors which include dichlorosilane ortrichlorosilane.

[0037] The benefits of the present gap-filling dielectric material maybe understood in terms of the microscopic transformations to thematerial taking place during cure step 16, principally the processes ofsintering and reflow. Sintering is conventionally defined as the processof heating and compacting a powdered material at a temperature below itsmelting point, or glass transition temperature, in order to weld theparticles together into a single rigid shape.

[0038] Reflow occurs at temperatures above the glass transitiontemperature and induces physical conformational changes. The reflowmechanism for PSG and BPSG materials is described by R. A. Levy (J.Electrochem. Soc., Vol 133, No. 7, pp. 1417 (1986) ). Reflow is drivenby surface tension forces. The forces are proportional to σ/R², where σis the surface tension and R is the radius of curvature. Forconventional BPSG reflow the radius of curvature is on the order of 100to 500 Å for conformal coating of narrow gap, whereas the radius ofcurvature between the particles in the porous film is on the order of 5to 50 Å, depending on particle size. The surface tension forces insidethe porous film can therefore be several orders of magnitude larger thanfor conventional coating, thus inducing reflow at even higherviscosities. Therefore, reflow can convert an initially open porestructure to a closed pore structure at a lower temperature (or shortertime at same temperature) than would be required for conventionalreflow.

[0039] For process 10, that does not include infiltration with a matrixmaterial, the deposited colloidal dispersion, after removal of thesolvent, consists essentially of the nanoparticles. In these cases,curing may induce sintering of the nanoparticles. Using nanoparticlescontaining phosphorous or boron dopants lowers the minimum temperatureneeded for sintering. Using doped nanoparticles also lowers the glasstransition temperature such that reflow of doped nanoparticles can occurat the furnace cure and RTP temperatures described above.

[0040] For processes 20 and 30, which include infiltration with matrixmaterials, the curing process can induce chemical changes in the matrixmaterial. For example, using the silicon-containing spin-on polymerslisted above for liquid phase infiltration, curing in an atmospherecontaining oxygen converts the matrix material to silica or siliconoxynitride. In addition, when the nanoparticles are composed of amaterial convertible to a dielectric, such as silicon, curing in anoxygen or nitrogen atmosphere induces chemical changes in thenanoparticles, as well. The reflow and sintering properties of thematrix material may be tuned through doping. For example using matrixmaterials containing phosphorous or boron lowers the reflow temperatureso that curing induces both chemical and physical change. In the presentcase, reflow occurs at temperatures lower than those required forconventional BPSG glasses. Without being bound to any theory, theinventors attribute the lower reflow temperature to the fact that thedriving force for reflow is surface energy, which is inverselyproportional to radius of curvature. For the colloidal dispersions ofnanoparticles of the present films, the relevant radius of curvature isAngstroms while it is hundreds of Angstroms for conventional BPSG reflowprocesses. In addition, in the present instance, the required flowdistance is more than an order of magnitude smaller.

[0041] Thus it may be understood that thermal processing modifies thegap-filling dielectric material, minimizing open porosity, reducing itsporosity, and, in some cases, increasing its density, by sinteringnanoparticles together and/or by inducing reflow in the nanoparticlesand/or in the matrix material. The higher the process temperature, thegreater the resulting modification to the pore structure. Reducing theopen porosity increases the resistance of the dielectric material tocommonly used buffered oxide etchant (BOE) solution, for example asolution containing ammonium fluoride and/or hydrogen fluoride. Asreported in the examples below, dielectric material formed from acolloidal dispersion of silica nanoparticles by thermal processingalone, process 10, and also formed from silica nanoparticles byinfiltration with boron doped hydrogen silsesquioxane, process 20, wasused to fill gaps in a patterned wafer. Both the infiltrated andnon-infiltrated material in the gaps were resistant to a standard 500:1BOE solution. High process temperatures are associated, however, withmaterial shrinkage which can lead to cracking or delamination ofmaterials in narrow gaps. Use of the dense nanoparticles in the presentmethods minimizes thermal shrinkage. For colloidal silica withoutinfiltration for example (see Example 2 below), after annealing at 900°C., total thickness shrinkage of only 4.5% with respect to the bakedfilm has been observed. Therefore, thermal processing temperatures canbe selected according to the present methods to provide gap-fillingdielectric materials that are both crack free and resistant to etchants.

[0042] The features and benefits of the present invention are furtherillustrated but not limited by the following experimental examples.

ANALYTICAL TEST METHODS

[0043] In characterizing experimental results, refractive index and filmthickness was measured using a Woollam Variable Angle SpectroscopicEllipsometer Model MMA. Film thickness samples were measured post-bake,post-cure, and post-anneal. Percentage shrinkage was calculated as thechange in film thickness divided by the post-bake thickness. Coatedpatterned wafers after bake were cleaved to reveal feature sizes. Thecross section was gold stained with a thin gold layer. Scanning ElectronMicroscope (SEM) images at magnifications ranging from 40,000 to 100,000were obtained using a JOEL JSM model 6330F SEM apparatus.

EXAMPLE 1 Preparation of Colloidal Dispersion of Silica

[0044] A 1.8 wt % colloidal silica dispersion in cyclohexanone wasprepared by combining 50.3 gm of a 10 wt % colloidal silica stocksolution in cyclohexanone (Catalyst and Chemical Industries Co, Japan)with 225 gm cyclohexanone. Average particle diameter of the colloidalsilica was 10.5 nm. Analysis of the metal concentration of the solutiongave: Ca: 4.8 ppb; Cr: <1 ppb; Cu: 4.2 ppb; Fe: <5 ppb; Mg: 0.7 ppb; Mn:<1 ppb; Ni: <0.5 ppb; K: <5 ppb; and Na: 16 ppb.

[0045] A 3.5 wt % colloidal silica dispersion in cyclohexanone wasprepared by mixing 90.6 gm of the 10% colloidal silica stock solutionwith 170 gm cyclohexanone. The solutions were homogenized by ultrasonicagitation for 30 minutes and then filtered through a 0.1 micron filterbefore being used in spin coating onto blanket or patterned wafers.

EXAMPLE 2 Coating of the Colloidal Silica Solution

[0046] The 1.8 wt % colloidal silica solution of Example 1 wasspincoated onto an 8 inch silicon blanket wafer, hereafter termed ablanket wafer, and onto a patterned wafer with feature sizes rangingfrom 50 nm to several microns with step heights of 0.2 to 0.4 μm. Thespin conditions included a dynamic dispense for 3 seconds at 300 rpmwith dispense volume of 2 ml, followed by final spin at 2000 rpm for 20seconds with acceleration of 50 rpm/sec. The coated wafers were baked at80° C., 150° C. and 250° C. for one minute each, cured, and annealed.During the cure, the temperature was ramped from 450° C. to 700° C. at arate of 5° C./min and then held at 700° C. for 30 minutes. Cureatmosphere was N₂, at 16 liter/min and O₂ at 4 liter/min. The waferswere annealed at 900° C. in flowing nitrogen for 20 minutes.

[0047] Film thickness and shrinkage of the silica film on the blanketwafer after each processing step are listed in Table 1. Crack-free clearfilms with no visible defects were obtained when deposited on blanketwafers. Film thickness was measured with a spectroscopic ellipsometer(J. A. Woolam VASE.) Film non-uniformity was less than 2%. TABLE 1 FilmThickness and Shrinkage of Silica Film Film Thickness Refractive IndexShrinkage from Bake Post-bake 491 Å 1.24 — Post-cure 485 Å 1.23 1.5%Post-anneal 478 Å 1.24 4.5%

[0048] A refractive index of 1.24 indicated that the deposited film hada porous silica structure. This refractive index value may be comparedwith a refractive index of 1.36 for a 20% porous silica film and with arefractive index of 1.45 for non-porous silica films. The porous filmsshowed very low shrinkage even after heat-treating to as high as 900° C.

[0049] The gap-filling ability of the colloidal silica in narrow gaps ofthe pattern wafers was determined from SEM pictures of the cleavedwafers. SEM pictures were taken for (i) the post-bake, (ii) post-cureand (iii) post-anneal wafers. All three SEM images showed that thesilica nanoparticles uniformly fill gaps size ranging from 50 nm toseveral microns in size. There were no delamination or cracking aftercure and after anneal and the nanoparticles also fill the comers aroundthe bottom of the gaps.

EXAMPLE 3 Preparation of Boron-Doped Silsesquioxane Solution

[0050] 8.2 gm of triethoxysilane, 0.60 gm of deionized water and 0.75 gmof 0.02 N nitric acid were added to 41 gm of acetone in a plasticbottle. The mixture was allowed to remain at room temperature for 18hours and then diluted with 50 gm of n-propoxy propanol and 20 gm ofdenatured ethanol to form a base silsesquioxane solution.

[0051] 3 gm of boron oxide (B₂O₃) was dissolved in 100 gm of isopropylalcohol to form a 3% boron oxide solution. 6.66 gm of the 3% boron oxidesolution was added to 100.55 gm of the base silsesquioxane solution. Themolecular weight of the boron-doped silsesquioxane resin was 1256 atomicmass units from gel permeation chromatography. The resin contained 2.0%boron by weight. The molecular weight of the resin after 15 days at roomtemperature was 1797.

COMPARATIVE EXAMPLE 4 Coating of Boron-Doped Silsesquioxane Solution

[0052] The boron doped silsesquioxane solution of Example 3 was coatedonto a 6″ blanket wafer and onto a pattern wafer using the spin, bake,and cure cycles of Example 2. Film properties were listed in Table 2.Post-bake film thickness of 1100 Å on the blanket wafer was obtainedfrom two coating steps with a bake cycle between the coating steps. Agood quality film was obtained on a blanket wafer. The FTIR spectrum ofthe cured film showed that the silane (SiH) evidenced in the post-bakefilm by peaks at 2000-2250 cm⁻¹ and 800-900 cm⁻¹ had been converted tosilica and as a result, a boron-doped silica film had been produced.TABLE 2 Film Thickness and Shrinkage of Boron-Doped Silsesquioxane FilmFilm Thickness Refractive Index Shrinkage from Bake Post-bake 1110 Å1.410 — Post-cure  868 Å 1.438 21.8% Post-anneal  800 Å —   28%

[0053] The gap filling behavior of the material was observed from SEMpictures taken after each processing step. The boron-dopedsilsesquioxane filled gaps in the patterned wafer without void ordelamination down to the narrowest size of about 50-60 nm after thepost-bake. After a 700° C. cure in nitrogen/oxygen, the silica formedshowed no delamination, but a lower density material was identifiedaround the lower corners of the gap, based on SEM brightness of thatregion. After a 900° C. anneal in nitrogen, delamination and crackingoccurred due to further shrinkage induced in the annealing.

EXAMPLE 5 Infiltration of Colloidal Silica Film with Boron-DopedSilsesquioxane

[0054] The 1.8 wt % colloidal silica solution from Example 1 was used tocoat a patterned wafer, following the spin and bake conditions ofExample 2. After bake, the boron-doped silsesquioxane solution fromExample 3 was spin coated onto the coated patterned wafer using thespin/bake sequences of Example 4. The infiltrated wafer was then curedand annealed using the cure and anneal conditions of Example 2.

[0055] The extent of gap-filling in the patterned wafer was determinedby examining the SEM pictures of the gaps of various sizes. For all thepost-bake, post-cure and post-anneal samples, there were no delaminationinside the gap of all sizes down to the narrowest size of about 50-60nm. An overlayer of boron-doped silica was formed on top of theinfiltrated colloidal silica film. The thickness of the overlayer wasabout 200 nm.

EXAMPLE 6 Etching Resistance

[0056] Silica coated blanket wafers of Example 2 boron-dopedsilsesquioxane coated wafers of Example 4 were dipped into 500:1 BOE(buffered oxide etchant) solution containing ammonium fluoride for 180seconds at an etch temperature of 21° C. After etching, the treatedwafers were rinsed with deionized water and the film thickness wasmeasured. The etch rate of silicon oxide produced by CVD deposition ofTEOS was used as a reference for comparison. Etch rate was calculated asthe decrease of film thickness divided by the etch time. Etch rate inÅ/sec and relative etch rate with respect to that of CVD TEOS werelisted in Table 3. The average etch rate of the CVD TEOS was about 0.45Å/sec. TABLE 3 Etch Rate of Colloidal Silica and Boron-DopedSilsesquioxane Films Etch Rate Etch Rate relative to (Å/sec) CVD oxidePost-cure Colloidal Silica 7.0 15 Post-anneal Colloidal Silica 1.7 3.8Post-cure B-silsesquioxane 0.66 1.5

[0057] Coated patterned wafers of Examples 2, 4, and 5 were dipped in500:1 BOE solution for 20 seconds, followed by a deionized water rinse.SEM pictures of the etched samples were taken to examine the etchresistance of the dielectric filled in the narrow gaps. Results aresummarized below. TABLE 4 Etching of Patterned Wafers Heat TreatmentEtching of Gap Dielectric Example 2 Post-cure no crack, no etchingExample 2 Post-anneal no crack, no etching Example 4 Post-cure severelyetched Example 4 Post-anneal crack before etching, etched out Example 5Post-cure no crack, no etching Example 5 Post-anneal no crack, noetching

[0058] The inclusion of silica nanoparticle in the dielectric has showntwo major benefits: (1) elimination of cracking or delamination inducedby 700-900° C. heat treatment and (2) significant improvement in theetch resistance of the gap dielectric to BOE etching. These benefits aredue to the low shrinkage and high density achieved by using the inertfiller composite approach.

EXAMPLE 7 Synthesis of Doped Silicates

[0059] To 10.0 gm of a 5% solution of P₂O₅ in 2-propanol, 0.38 gm B₂O₃was dissolved with stirring for 1 hour. 9 gm TEOS in 9.5 gm acetone wasadded followed by the addition of 1.38 gm 1N HNO₃ and 0.5 gm water. Themixture was heated to boiling around 66° C. and maintained for 3 hours.The molecular weight was about 2000 amu. The silicate resin wasestimated to have 8% by weight phosphorus and 4% by weight boron. Thesolid content of the solution was 8 wt %.

EXAMPLE 8 Infiltration of Colloidal Silica Film with Doped Silicate

[0060] A colloidal silica film was deposited onto a blanket wafer usingthe 1.8 wt % colloidal silica solution from Example 1. The spin recipewas 5 seconds at 300 rpm for dynamic dispense of 2 ml of solutionfollowed by a 20 second spin at 2000 rpm. The wafer was baked at 80° C.,150° C. and 250° C. for one minute each. The film thickness was 520 Åwith a refractive index of 1.23.

[0061] The porous silica film was infiltrated with the doped silicatesolution from Example 7. A rinsing solution consisting of 50 wt %2-propanol and 50 wt % acetone was prepared. The following infiltratingand rinsing procedure was used: (1) dynamic dispense of 4 ml of dopedsilicate solution onto the wafer at 300 rpm for 5 seconds; (2) spread ofliquid at 1000 rpm for 1 second; (3) rest for 5 seconds; (4) dynamicrinse using 4 ml of rinsing solvent for 5 seconds at 300 rpm; and (5)final spin at 2000 rpm for 20 seconds.

[0062] The infiltrated film was baked at 80° C., 150° C. and 250° C. forone minute each. Film thickness after bake was 517 Å with refractiveindex of 1.43. The increase of refractive index to the value of 1.43showed that a solid non-porous film was been produced.

EXAMPLE 9 Infiltration of Colloidal Silica Film with Doped Silicate

[0063] The 1.8 wt % colloidal silica solution of Example 1 was used tocoat a patterned wafer using a dynamic dispense of 3 ml in 3 seconds at300 rpm, followed by a final spin of 2000 rpm for 20 seconds. One minuteafter spin without baking, 3 ml of doped silicate solution from Example7 was dispensed onto the wafer in 3 seconds while spinning at 300 rpm,followed by final spread at 2000 rpm for 20 seconds. The infiltratedfilm was then baked, cured and annealed according to the recipedescribed in Example 2. SEM images of the annealed film showed nocracking of the film in the gaps and in the top layer of the infiltratematerial. The infiltrated wafer was soaked in BOE 500:1 solution for 20seconds. No etching was observed around the corners of the wide andnarrow gaps in SEM pictures at 80000 magnification.

EXAMPLE 10 Premixing of Colloidal Silica and Doped Silicate Solutions

[0064] A 4 wt % solution of colloidal silica was prepared by adding 23grams of cyclohexanone to 15 grams of 10% colloidal silica stocksolution. 9 grams of TEOS was mixed with 9.5 grams of acetone, followedby the addition of 1.38 g 1N nitric acid and 0.58 grams of deionizedwater. 10 grams of5 wt % P₂O₅ solution in isopropyl alcohol (IPA) wasadded and mixed well. The mixture was stirred for 72 hours at roomtemperature. The solution was then diluted with 15.23 gm of acetone and15.23 gm of IPA to give a final solid content of 4 weight percentP-doped silicate.

[0065] The following three solutions were prepared and spin-coated ontoblanket wafers, followed by bake, cure and anneal as Example 2:

[0066] Solution 10A: 10 grams of 4% silica and 10 grams of 4% P-dopedsilicate solution

[0067] Solution 10B: 16 grams of 4% silica and 4 grams of 4% P-dopedsilicate solution

[0068] Solution 10C: 18 grams of 4% silica and 2 grams of 4% P-dopedsilicate solution

[0069] Results are shown in Table 5 below. TABLE 5 Premixing ofColloidal Silica and P-Doped Silicate Solutions Film Shrinkage Etch RateProcess SiO₂: Thick- Refractive from in 500:1 Solution (° C.) Silicateness Index bake BOE 10A 700 1:1 798 Å 1.440 8.5% 1.46 Å/sec 10A 900 1:1705 Å 1.441 19.2% 0.58 Å/sec 10B 700 4:1 750 Å 1.380 5.6% 0.88 Å/sec 10B900 4:1 699 Å 1.388 12.2% 0.61 Å/sec 10C 700 9:1 805 Å 1.270 3.3% — 10C900 9:1 776 Å 1.280 6.8% —

[0070] Although the present invention has been described in terms ofspecific materials and conditions, the description is only an example ofthe invention's application. Various adaptations and modifications ofthe processes disclosed are contemplated within the scope of theinvention as defined by the following claims.

We claim:
 1. A process of forming a dielectric layer on a substrate, theprocess comprising: depositing a colloidal dispersion on a substrate,the colloidal dispersion comprising particles of a dense materialdispersed in a solvent, wherein the dense material is a dielectricmaterial or a material convertible to a dielectric material by oxidationor nitridation; drying the colloidal dispersion to form an intermediatelayer; infiltrating the intermediate layer with a matrix material toform an infiltrated layer; and curing the infiltrated layer whereby theinfiltrated layer is modified forming the dielectric layer.
 2. Theprocess of claim 1 wherein infiltrating the intermediate layer with amatrix material is infiltrating the intermediate layer with a coatingsolution of a spin-on polymer material.
 3. The process of claim 2wherein the spin-on polymer material comprises a material selected fromthe group consisting of silicates, hydrogen silsesquioxanes,organosilsesquioxanes, organosiloxanes, silsesquioxane-silicatecopolymers, silazane-based materials, polycarbosilanes, andacetoxysilanes.
 4. The process of claim 2 wherein the spin-on polymermaterial comprises a species selected from the group consisting ofarsenic, antimony, phosphorous, and boron.
 5. The process of claim 1wherein the particles of the dense material are particles comprising amaterial selected from the group consisting of silica, silicon, siliconnitride, silicon oxynitride, aluminum, aluminum nitride, and aluminumoxide.
 6. The process of claim 5 wherein the dense material furthercomprises a species selected from the group consisting of arsenic,antimony, phosphorous, and boron.
 7. The process of claim 5 wherein theparticles of the dense material comprise silica.
 8. The process of claim5 wherein the particles of the dense material comprise silicon.
 9. Theprocess of claim 1 wherein the particles have a characteristic dimensionbetween about 2 nanometers and about 50 nanometers.
 10. The process ofclaim 1 wherein the dielectric layer is a pre-metal dielectric layer onan integrated circuit device.
 11. The process of claim 1 wherein thedielectric layer fills a trench in a shallow trench isolation structure.12. The process of claim 1 wherein the dielectric layer fills gaps ofdimension less than 100 nanometers.
 13. The process of claim 12 whereinthe dielectric layer is heated above 700° C. without cracking.
 14. Theprocess of claim 1 wherein the dielectric layer is resistant to standardbuffered oxide etchant solutions.
 15. The process of claim 1 whereininfiltrating the intermediate layer with a matrix material is depositinga matrix material on the intermediate layer by chemical vapordeposition.
 16. The process of claim 1 wherein infiltrating theintermediate layer with a matrix material is depositing a matrixmaterial on the intermediate layer by atomic layer deposition.
 17. Theprocess of claim 15 wherein the matrix material is selected from thegroup consisting of phosphosilicate glass, borosilicate glass, andborophosphosilicate glass.
 18. The process of claim 1 wherein curing theinfiltrated layer whereby the infiltrated layer is modified is curingthe infiltrated layer whereby open porosity of the infiltrated layer isminimized.
 19. A process of forming a dielectric layer on a substrate,the process comprising: depositing a colloidal dispersion on asubstrate, the colloidal dispersion comprising particles of a densematerial dispersed in a solvent, wherein the dense material is adielectric material or a material convertible to a dielectric materialby oxidation or nitridation; drying the colloidal dispersion to form anintermediate layer; curing the intermediate layer whereby theintermediate layer is modified everywhere on the substrate, forming thedielectric layer.
 20. The process of claim 19 wherein the particles ofthe dense material are particles comprising a material selected from thegroup consisting of silica, silicon, silicon nitride, siliconoxynitride, aluminum, aluminum nitride, and aluminum oxide.
 21. Theprocess of claim 19 wherein the dense material further comprises aspecies selected from the group consisting of arsenic, antimony,phosphorous, and boron.
 22. The process of claim 19 wherein theparticles of the dense material comprise silica.
 23. The process ofclaim 19 wherein the particles have a characteristic dimension betweenabout 2 nanometers and about 50 nanometers.
 24. The process of claim 19wherein the dielectric layer is a pre-metal dielectric layer on anintegrated circuit device.
 25. The process of claim 19 wherein thedielectric layer fills a trench in a shallow trench isolation structure.26. The process of claim 19 wherein the dielectric layer furthercomprises a spin-on polymer.